The present invention relates to the calibration of a quadrature demodulator.
As an example of a synchronous detector, a quadrature demodulator synchronously detects two baseband signals, i.e., an I signal (in-phase component) and a Q signal (quadrature component) that are in quadrature with each other, such as signals modulated by QPSK (Quadrature PSK). A specific example of a quadrature demodulator 1 will be described with reference to FIG. 26.
First, a frequency conversion section 10 accepts at its input a received signal. The section 10 converts the received signal into an intermediate frequency signal IF of a predetermined frequency and supplies this signal IF to an I signal conversion section 100 and a Q signal conversion section 200. The I signal conversion section 100 receives a local signal of a frequency LF1 from a reference oscillator 40, mixes the local signal with the intermediate frequency signal IF, and generates an I signal (in-phase component). The Q signal conversion section 200 receives from the reference oscillator 40 a local signal of the same frequency LF1 with its phase 90 degree shifted by a 90 degree phase shifter 42, mixes such a local signal with the intermediate frequency signal IF, and generates a Q signal (quadrature component). In response to the I signal (in-phase component) and the Q signal (quadrature component), a succeeding circuit 90 performs a desired operation.
Here, factors such as temperature changes cause errors in the I signal conversion section 100 and the Q signal conversion section 200. These errors are, e.g., amplitude, phase, and offset errors. These errors must be removed, or the sections 100 and 200 must be calibrated, since these errors adversely affect the operation of the succeeding circuit 90.
The following method is available to calibrate the I signal conversion section 100 and the Q signal conversion section 200. The user feeds a calibration signal of a predetermined frequency to the I and Q signal conversion sections 100 and 200. At this point, the quadrature demodulator 1 must stop processing the received signal and adjusts attenuation values and the like of variable attenuators, variable delay units and the like in the I and Q signal conversion sections 100 and 200 to calibrate the sections 100 and 200.
That is, during calibration, the user is not allowed to supply a signal to the quadrature demodulator 1, or the quadrature demodulator 1 must stop its reception.